Referring more particularly to the case of an optical sensor, used in X-ray imaging, in order to improve the contrast of the images obtained, it is useful to have a large capacitor on the pixel electrodes in order to store a large amount of charge since, the higher the charge stored, the better the image contrast. Currently, and as shown in FIG. 1, a large storage capacitor is obtained by producing a buried ground plane and a capacitive electrode on the gate's level. More precisely, a layer of a metal such as titanium is deposited on a substrate 1 for the purpose of forming a ground plane 2. This titanium layer is etched so as to obtain the capacitive electrode 2. Next, an insulation layer 3, made of a material such as silicon oxide SiO2, with a thickness of 2000 Å for example, is deposited in a known manner on this capacitive electrode 2. A titanium/molybdenum metal bilayer is then deposited on the insulating layer, which bilayer is then etched so as to produce the gate 4 of the thin-film transistor as well as a second electrode 5 which forms a storage capacitor with the electrode 2. Next, an insulating layer 6 made of silicon nitride SiN, having for example a thickness of 3000 Å, is deposited in a known manner, followed by a layer 7 of semiconductor material such as, for example, amorphous silicon. This layer 7 is etched in a known manner. Next, a metal layer, for example made of molybdenum, is deposited and etched so as to produce the first electrode 8 or drain and the second electrode 9 or source of the thin-film transistors. The source electrode 9 is connected in a known manner at 10 to the capacitive electrode 5. Once the source and drain 8, 9 have been produced, a layer 11 of an insulating material such as silicon nitride SiN is deposited with a larger thickness, for example about 5000 Å. A connection hole 13 is then produced down to the source 9, the role of which will be explained later. Next, a transparent metal layer, such as a layer of ITO (indium titanium oxide), is deposited in a known manner on the SiN insulating layer so as to produce the pixel electrode 12. This pixel electrode is connected via the hole 13 to the source 9 in order to operate in a known manner. In this case, the storage capacitor is obtained between the electrode 5 and the electrode 2 forming the ground plane. However, to produce this storage capacitor, it is necessary to have two additional masking levels paired with a standard active matrix production process. In addition, the insulator of the capacitor, namely the insulating layer 3, must be thin so as to guarantee the highest possible storage capacitance.